1. Field of the Invention
This invention is directed to novel liquid block copolymers having unsaturation only on the terminal blocks and methods of preparation thereof. More particularly, the invention is directed to liquid block copolymers comprising triblock units wherein the middle block of each triblock unit is substantially selectively hydrogenated and therefore contains substantially no unsaturated groups, while each of the terminal blocks of each triblock unit contains a sufficient amount of unsaturation for curing the block copolymers.
The invention is also directed to random liquid copolymers which, when selectively hydrogenated, contain molecules having substantially saturated backbones and random, pendant unsaturation.
The invention is additionally directed to chemically modified derivatives of the above block and random copolymers.
Crosslinking of the polymers of the invention produces elastomeric vulcanizates having unusual properties, e.g., high elongation and excellent aging characteristics.
2. Description of Related Art
Elastomers (or rubbers) of either natural or synthetic origin usually require vulcanization for transformation into insoluble, high strength elastomeric products. Before vulcanization, rubbers possess inferior properties and low strength which limit their utility.
There are a number of well known methods for achieving the vulcanization, also referred to as crosslinking, of unsaturated elastomers. Such methods include the use of sulfur and accelerators, peroxides, benzoquinone dioxime, certain phenolic resins and similar agents. Any of the above or any other well known vulcanizing techniques may be utilized to crosslink the elastomers of this invention.
Liquid elastomers are well known and are used in various applications. For example, many functionally terminated polybutadiene liquid elastomers are known. These materials are generally highly unsaturated and frequently form the base polymer for polyurethane formulations. The preparation and application of hydroxy-terminated polybutadiene is detailed by J. C. Brosse et al in HYDROXYL-TERMINATED POLYMERS OBTAINED BY FREE RADICAL POLYMERIZATION-SYNTHESIS, CHARACTERIZATION AND APPLICATIONS, ADVANCES IN POLYMER SCIENCE 81, Springer-Verlag, Berlin Heidelberg, 1987, pp 167-220.
Also, liquid polymers possessing acrylate, carboxy - or mercapto-terminals are known. In addition to butadiene, it is known to utilize isoprene as the base monomer for the liquid elastomers. The liquid elastomers may contain additional monomers, such as styrene or acrylonitrile, for controlling compatibility in blends with polar materials, such as epoxy resins.
Also known in the prior art are pure hydrocarbon, non-functionalized liquid rubbers. These liquid elastomers contain varying degrees of unsaturation for utilization in vulcanization. Typical of highly unsaturated liquid elastomers is polybutadiene, e.g., that sold under the name RICON by Colorado Chemical Specialties Co. A liquid polyisoprene which has been hydrogenated to saturate 90% of its original double bonds is marketed as LIR-290 by Kuraray Isoprene Chemical Co. Ltd. Still more highly saturated are liquid butyl rubbers available from Hardman Rubber Co., and Trilene, a liquid ethylene-propylene-diene rubber (EPDM) from Uniroyal Chemical Co. The more highly saturated liquid elastomers exhibit good oxidation and ozone resistant properties. The above prior art liquid elastomers, with either high or low levels of unsaturation, are characterized in that, having random unsaturation, they are randomly crosslinked during vulcanization. The success of vulcanization in incorporating all molecular chains into the final crosslinked network with minimal "loose ends" is termed the degree of network perfection. An imperfect network, wherein crosslinks occur randomly and sometimes not near the end of a molecular chain, produces a vulcanized polymer having poor mechanical and elastomeric properties caused by chain ends which are not a part of the tightly bound network. In order to insure the highest degree of network perfection attainable, randomly unsaturated elastomers must be crosslinked extensively. The large number of crosslinks necessary dictates that the average distance between crosslinks (M.sub.c) must be relatively small in comparison with the dimensions of the whole molecule. Elastomeric properties, such as elongation, depend greatly on M.sub.c --the smaller the M.sub.c the worse are the elastomeric properties, e.g., the lower the elongation of the polymer.
Falk, JOURNAL OF POLYMER SCIENCE: PART A-l, Volume 9, 2617-2623 (1971), the entire contents of which are incorporated herein by reference, discloses a method of selectively hydrogenating 1,4,-polybutadiene in the presence of 1,4-polyisoprene. More particularly, Falk discloses selective hydrogenation of the 1,4-polybutadiene block segment in the block copolymer of 1,4-polybutadiene-1,4-polyisoprene-1,4-polybutadiene and in random copolymers of butadiene and isoprene, with both polymerized monomers having predominantly 1,4-microstructure. Selective hydrogenation is conducted in the presence of hydrogen and a catalyst made by the reaction of organoaluminum or lithium compounds with transition metal salts of 2-ethylhexanoic acid.
Falk, DIE ANGEWANDTE CHEMIE 21 (1972) 17-23 (No. 286), the entire contents of which are also incorporated herein by reference, discloses the selective hydrogenation of 1,4-polybutadiene segments in a block copolymer of 1,4-polybutadiene-1,4-polyisoprene-1,4-polybutadiene.
Hoxmeier, Published European Patent Application 88202449.0, filed on Nov. 2, 1988, Publication Number 0 315 280, published on May 10, 1989, discloses a method of selectively hydrogenating a polymer made from at least two different conjugated diolefins. One of the two diolefins is more substituted in the 2,3 and/or 4 carbon atoms than the other diolefin and produces tri- or tetra-substituted double bond after polymerization. The selective hydrogenation is conducted under such conditions as to hydrogenate the ethylenic unsaturation incorporated into the polymer from the lesser substituted conjugated diolefin, while leaving unsaturated at least a portion of the tri- or tetra-substituted unsaturation incorporated into the polymer by the more substituted conjugated diolefin.
Mohajer et al, Hydrogenated Linear Block Copolymers of Butadiene and Isoprene: Effects of Variation of Composition and Sequence Architecture on Properties, 23 POLYMER 1523-1535 (September 1982) disclose essentially completely hydrogenated butadiene-isoprene-butadiene (HBIB), HIBI and HBI block copolymers in which butadiene has predominantly 1,4-microstructure.
Kuraray K K, Japanese published patent application Number JP-328 729, filed on Dec. 12, 1987, published on Jul. 4, 1989, discloses a resin composition comprising 70-99% wt. of a polyolefin (preferably polyethylene or polypropylene) and 1-30% wt. of a copolymer obtained by hydrogenation of at least 50% of unsaturated bond of isoprene/butadiene copolymer.
Heretofore, the art has failed to produce liquid hydrocarbon elastomers having the capability of maintaining relatively large distance between cross-links (high M.sub.c) after vulcanization.
Accordingly, it is an object of this invention to provide liquid polymers capable of being vulcanized to a substantially perfect network with a distance between crosslinks nearly equivalent to the dimensions of the unvulcanized elastomeric molecule. In addition to the expected improvements in elastomeric properties, the unperturbed saturated main chain of the polymers of this invention provides a high degree of oxidative and thermal stability. Unique materials can also be obtained by chemical modification of the polymers of this invention since the polymers of the invention can be selectively modified at the terminal ends of the molecules.
It is an additional object of this invention to provide a method for the production of random copolymers having controlled amounts of unsaturation incorporated randomly in an otherwise saturated backbone. In contrast to EPDM, the level of unsaturation can be inexpensively and easily controlled, e.g., from 1% to 50%, to provide a wide variation in vulcanization rate and potential co-curability with various highly unsaturated rubbers based on butadiene or isoprene.